Kinjutsu

Orochimaru using a kinjutsu: the Summoning: Impure World Reincarnation.

Kinjutsu (禁術, Literally meaning: Forbidden Techniques) are techniques that have been banned from being taught or used.

This ban could have been put in place for any number of reasons, but kinjutsu can generally be put in either of three categories:

Techniques that cause harm to the user themselves, such as the Wind Release: Rasenshuriken, which the mere use of is both highly useful as well as detrimental to the user. Although, with training, a user can overcome this weakness by compensating for it, enabling them to use the technique without any consequences, such as Naruto's throwing of the Rasenshuriken to avoid damaging himself. However, there are some that cannot avoid self-harm/sacrifice, such as Dead Demon Consuming Seal or Creation Rebirth.

Certain techniques that are known to cause massive collateral damage, such as the total destruction of a village and end with the death of everyone in it, thus the great moral ramifications of its potential lead many to labelling it as a forbidden technique (e.g. the Fury technique, which appeared only in the anime).

Despite being banned, the mere use of these techniques generally does not carry any legal consequences in respects to the village they're used in, except those brought on by the techniques themselves. However, their use is heavily frowned upon, especially those techniques that violate nature. On the other hand, researching on and creating kinjutsu are punishable, as Orochimaru and Hiruko were both forced to flee Konohagakure after being caught using live people as guinea pigs, while Deidara was forced to leave Iwagakure when he stole a kinjutsu to develop his passion for art.

Trivia

Despite not being an actual technique, the Eight Gates has been called a forbidden technique.[1]